Maintaining Makeup-Air Units

Effective maintenance of makeup-air units requires regularly scheduled inspections. The unit should be given a top to bottom inspection twice a year, usually at the start of the cooling season and at the start of the heating season. Be certain that the maintenance technician is familiar with the type of equipment you have installed. Direct-fired units are still relatively new and not all service contractors have experience with them.

The first step is to lock out the gas and electrical power to the unit. This should be obvious, but is often neglected.

Belt drives should be checked on a regular basis for wear, tension, alignment, and dirt accumulation. Alignment can be checked by using a straightedge long enough to span across both sheaves. Both edges of both of the sheaves should make contact with the straightedge.

Tension of the belt should be checked according to the manufacturer's instructions. Too much tension can cause premature failure of the fan and/or motor bearings. Too little tension causes squealing, slippage and overheated sheaves. If you have to replace the belts, make sure to move the motor enough that you can lift the belt off both sheaves without stretching the belt or putting stress on the bearings. If you have to replace a belt on multiple groove drives, replace all the belts at the same time to provide uniform drive loading.

Motor maintenance is usually confined to cleaning and lubrication if the motor is supplied with grease fittings. If there are no grease fittings, that is a subtle hint that the motor does not need to be lubricated. In fact, trying to lubricate such a motor will cause you more problems than the ones you thought you would solve. Removing dirt and grease build-up helps keep the motor cool, however this cleaning should be done with a dry rag. Never spray motors with solvents, steam, or water.

Fan wheels seldom need attention. However when the unit is operating in a dirty environment, oil and dirt may accumulate on the wheel and housing and may cause an imbalance. When this happens, a thorough cleaning of the fan assembly is required.

Bearings, again, when they are supplied with grease fittings, should be lubricated in accordance with the manufacturer's recommendations.

If the unit is going to be inactive for an extended period of time, lubricate the bearings just prior to shutdown. The fan should be rotated monthly to help prevent corrosion. If the unit has been idle for more than three months, purge the bearings with new grease before startup.

If the unit has cooling or heating coils, they should be inspected for corrosion and/or leaks. Always check to make sure the coil's air path is clear. If the coil is dirty, cleaning should be accomplished from the leaving air side so that dirt is not pushed farther into the coil.

On the gas line, remove the drip leg and remove any liquid or dirt that has accumulated, then reinstall it.

If the unit is an indirect-fired type, check the flue to make sure it is clear of debris. Also check the heat exchanger for cracks. If any cracks are found, the heat exchanger must be replaced before the unit is put into operation. Failure to do so could result in carbon monoxide and other flue gases being blown into the occupied space.

Check the burner to see that the orifices are clear of any dirt or debris. This may require removing the manifold and burner assembly. Remove any soot with a wire brush.

Clean the orifices with aerosol degreaser or compressed air. The use of a degreaser will retard the buildup of dirt. If you have to drill out the orifices be certain to use the proper drill-bit. If the instructions say to use a No. 47 bit, do not believe that a No. 48 bit is close enough. It isn't.

A member of HPAC Engineering's Editorial Advisory Board, Robert W. Tinsley PE, CFPS, CIAQP, has authored numerous articles for HPAC Engineering on the topics of insulation, sound attenuation, and commissioning. He can be reached atbtinsley@p2rscos.com.